1. Field:
This invention relates to sterilization and is specifically directed to the sterilization of heat-labile, medical and dental devices, instruments, and materials.
2. State of the Art:
Rapid recirculation of packaged or loose instruments is a highly desired objective of modern medical and dental practices. Methods for sterilizing heat-labile medical instruments are known, but none of the methods currently practiced fulfills all major requirements. Ethylene oxide gas treatment provides adequate sterilization, but requires a lengthy aeration period following treatment, in order to remove absorbed chemicals from porous materials. Instruments packaged in accordance with modern techniques cannot be submerged in liquids such as glutaraldehyde. Liquid sterilization of unpackaged instruments imposes the requirements of post-treatment rinsing and direct handling, both of which carry the risk of recontamination. Formaldehyde gas has been utilized as a disinfectant. It has long been understood, however, that formaldehyde gas is practical as a sterilant only at elevated temperatures and pressures. It is generally understood that sterilization by most methods is enhanced by increasing the temperature of the sterilization medium. The necessity for relatively low temperatures thus complicates the problem of sterilizing heat-labile instruments. It is also generally recognized that the efficacy of gas sterilization is characteristically decreased at lower pressures. The construction of high pressure vessels is relatively expensive, however, and the long term operation and maintenance of such vessels can be costly.
Gunnar Nordgren reported certain work done with gaseous formaldehyde under so-called "pulsing" conditions. Nordgren applied a vacuum to a sterilization chamber and periodically introduced air passed through aqueous formaldehyde (formalin) into the chamber. Each introduction of gas may be regarded as a "pulse." The pressure conditions within the chamber thus fluctuated between an established upper value and a reduced level. According to the procedure used by Nordgren, a vacuum pump was continuously operated so that as soon as formaldehyde gas was introduced to the chamber, the pressure within the chamber commenced its downward cycle. Nordgren found that repeated passages of air through heated formalin (37% by volume formaldehyde in aqueous solution) into the evacuated chamber warmed to 60.degree. C. was effective in destroying spores of Bacillus anthracis or Bacillus subtilis dried onto rubber. This work, as well as a description of related investigations up through the year 1939, is contained in a historical review, "Investigations on the Sterilization Efficacy of Gaseous Formaldehyde," by Gunnar Nordgren, publisher Einar Munksgaard, Copenhagen, 1939.
The biological indicators required under modern practice by the Environmental Protection Agency in its "Licensure Protocol" are set forth, for example, in Sporicidal Test (4) of the "Official Methods of Analysis" of the Association of Official Analytical Chemists, Washington, D.C., 13th Ed., 1980, Horowitz, AOAC Methods (1980). These indicators are considerably more difficult to sterilize with formaldehyde gas than are the aforementioned biological indicators utilized by Nordgren. The techniques taught by Nordgren have not demonstrated acceptable sporicidal activity of formaldehyde gas in accordance with AOAC Methods (1980).
There remains a need for an effective method for sterilizing heat-labile and other packaged instruments for which high temperature or liquid sterilization techniques are impractical or otherwise unsuitable. Although other sporicidal gasses are known, the sterilizing gas of choice is formaldehyde because of its familiarity, availability, and relative safety.